My dealer has not been good about topping off fluids lately. I bought some antifreeze (http://amzn.com/B0089LSFJ0) to get the level to just above minimum, but I'm concerned that I don't know what the dealer has put in there.

The antifreeze I got is supposed to be correct for my car; is it safe to add it with the existing fluid or should I ask the dealer what kind they've been using before I mix this in?

2 Answers
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If you want to know for sure, you can ask the dealer what they use. It would be good to know for the future anyway.

There are two main types of anti freeze, generally they all use ethylene glycol as the coolant base; though they differ in the type of corrosion inhibitors used. "Traditional" coolants (often green or yellow) generally use silicates, while "new style" (generally orange or pink) coolants use organic acids. GM's DexCool is an example of a newer organic acid coolant. These two types shouldn't be mixed if possible, as the effectiveness of the corrosion inhibitors can be reduced.

Unfortunately, checking the color of the existing coolant in the expansion tank isn't good enough to confirm a match against what you bought.

That said, if you merely need to top up the expansion bottle, you may be able to get away with adding only distilled water, which would be compatible with any coolant. Check the coolant concentration with a hydrometer, and if you've got sufficient coolant concentration, you can simply add water.

For what it's worth, apparently someone from the SAE (Society of Automotive Engineers) said you can safely mix up to 15% of the traditional coolant type into an organic acid type coolant without much of an effect on the corrosion inhibitors. I'd still avoid it if possible. I've personally seen brown sludgey coolant resulting from mixing traditional coolant into VW/Audi's organic acid coolant.

If you want to top it off just once, it's better to get a bottle of distilled water and pour that in. If you're an unlucky person, the two types of antifreeze might coagulate (form a jelly) and mess up your engine.

Anti-freeze is cheap enough for you to flush your coolant system and fill it up with the type you want to put in. I would suggest emptying the radiator, pouring some radiator flush chemicals into the system and follow the directions on the box. When that's done, fill it up with your new anti-freeze and "burp" your car. The directions should also be right on the bottle, but it's basically letting your car idle for 10-20 minutes with the AC set to hot and the radiator cap off.